OSNR Model For Optical Engineering Rules Used In A Planning Tool

ABSTRACT

Increasing data rates in next-generation optical networks requires a change in the type of optical modulation used to encode optical signals carried by the optical networks. Different types of optical modulation incur different optical impairments, which may degrade the fidelity of the optical signals by reducing the optical signal-to-noise ratio (OSNR). A method or corresponding apparatus in an example embodiment of the present invention provides a planning tool for deploying an optical network in a manner based on the optical modulation that reduces the cost and complexity of the deployed network. In one embodiment, the disclosed planning tool may adjust a model of the optical network to be deployed by changing the topology and/or the number and/or type of optical network elements in response to optical impairments for a given optical modulation.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.12/544,571 filed on Aug. 20, 2009 which claims the benefit of U.S.Provisional Application No. 61/218,146, filed on Jun. 18, 2009. Theentire teachings of the above applications are incorporated herein byreference.

BACKGROUND

Deploying optical network elements (ONEs) to form an optical network isa difficult and expensive proposition: network providers need tocorrectly anticipate customer demand while building reliable networks asinexpensively as possible. In addition, network providers must alsoanticipate future technological developments, such as increased datarates, to simplify network upgrades. In part, network providers attemptto minimize cost and reduce network complexity by deploying ONEs, suchas optical amplifiers and optical regenerators, in a way that minimizesthe required power while ensuring signal fidelity.

In digital communication schemes, such as those employed in opticalnetworks, signal fidelity may be characterized by a bit error rate(BER). Simply put, the BER is how frequently a receiver detects a bitincorrectly, that is, how often the receiver mistakes a representationof a logical ‘1’ for a representation of a logical ‘0’ or vice versa.Lower BERs are better; ideal (i.e., noise-free) receivers operate withBERs of zero (0), but shot noise and thermal noise at real receiverscause bit detection errors, raising BERs to measurable levels.

Currently, the target BER for optical networks is on the order of 10⁻¹².To meet the target BER, network providers must guarantee a minimumoptical signal-to-noise ratio (OSNR) at the receiver. The OSNR isusually defined as the ratio of the optical signal power P_(s) to theoptical noise power P_(n) in a given channel bandwidth,

$\begin{matrix}{{OSNR} = {10 \cdot {{\log_{10}\left( \frac{P_{s}}{P_{n}} \right)}.}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

For digital signals, the detected power switches between a high leveland a low level at a given data, or bit rate. In optical networks, thehigh and low levels can be defined in terms of a number of photons: forexample, a 5 mW, 40 GHz optical signal in the Wavelength DivisionMultiplexing (WDM) C band may have a corresponding high level of about10⁶ photons and a low level of 0 photons. In a shot-noise limitedreceiver, a signal of 10⁶ photons has an OSNR of 30 dB.

Because bits can be defined in terms of photons, the bit rate can bedefined in terms of photons per second. As the bit rate increases, thenumber of photons per bit decreases given a constant optical power(i.e., spreading a constant number of photons per second over a largernumber of bits per second reduces the photons per bit). The increasedbit rate also leads to a decreased OSNR—the bandwidth increases, but thesignal power remains constant, whereas the receiver noise powerincreases given a relatively constant noise power spectral density.Eventually, increasing the bit rate depresses the OSNR too far, pushingthe BER above acceptable levels. In optical networks that use directdetection (i.e., networks that use on/off keying), the BER is related tothe OSNR according to the relation

BER∝½·log₁₀ (OSNR),   Relation 2

where the OSNR is in linear units. As shown in Relation 2, maintaining aminimum BER requires maintaining a minimum OSNR. This, in turn, meansthat any increase in the bit rate should be offset by a correspondingincrease in the OSNR to keep the BER at acceptable levels.

As light propagates through a network, however, it is absorbed andscattered, reducing the signal power and the OSNR. In addition, signalspropagating through optical fiber suffer from loss due to four-wavemixing, chromatic dispersion, and polarization mode dispersion, furtherreducing the OSNR. As stated above, reductions in OSNR hamper thenetwork's ability to support higher bit rates.

In long-haul and metro optical networks, optical amplification may boostthe signal power to reliably detectable levels. Amplifiers add noise tothe signal, however, despite increasing the signal strength. Even idealamplifiers double the amount of noise present, which corresponds to areduction of the OSNR by 3 dB.

Optical regeneration restores degraded signals to detectable statususing optical-to-electrical-to-optical conversion. The degraded opticalsignals are converted to electrical signals, which can be processed inthe electrical domain before being converted back to the optical domain.The resulting optical signals may have high enough OSNRs to bedetectable throughout the network. Unfortunately, the transpondersrequired to regenerate optical signals are complex and costly. Worse,their complexity and cost increase with the data rate and the number ofchannels, making regeneration an unattractive option for maintainingOSNR throughout an optical network.

SUMMARY

Embodiments of the present invention include methods of and tools forplanning deployment of an optical network. First, a model of an opticalnetwork, which includes models representing a topology of opticalnetwork elements (ONEs), ONEs, and optical signals with opticalmodulation, is initialized. Next, optical signal-to-noise ratio (OSNR)penalties are computed as a function of the optical modulation and as afunction of optical impairments associated with the model of the opticalnetwork. Then the model of the optical network is iteratively adjustedin an attempt to change the OSNR penalties in a manner known to enabledetection of the optical signals to support communications between ONEs.Finally, indications of the model of the optical network are reportedafter a given number of iterations.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particulardescription of example embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingembodiments of the present invention.

FIG. 1 is a schematic diagram that illustrates a user using an exampleembodiment of the present invention for planning deployment of opticalnetwork elements.

FIG. 2 is a flow diagram of planning an optical network according to anexample embodiment of the present invention.

FIG. 3 is a flow diagram of planning an optical network according to analternative example embodiment of the present invention.

FIGS. 4 and 5 are block diagrams of example embodiments of an opticalnetwork deployment planning apparatus according to principles of thepresent invention.

DETAILED DESCRIPTION

A description of example embodiments of the invention follows.

The disclosed planning tool provides a method and correspondingapparatus for determining the placement and configuration of opticalnetwork elements (ONEs) in optical networks to guarantee a minimumoptical signal-to-noise ratio (OSNR) throughout the optical network,leading to a reduction in the number of transponders in the opticalnetwork. The example methods and apparatus disclosed herein may be usedto plan deployment of an optical network that has sufficiently highsignal strength to ensure reliable signal detection throughout theoptical network.

One measure of signal strength is the OSNR, defined as the ratio of theoptical signal power to the optical noise power in a given channelbandwidth. The OSNR may include effects of optical impairments, such asfour-wave mixing, polarization mode dispersion (PMD), chromaticdispersion, self-phase modulation, cross-phase modulation, and othermechanisms that may affect transmission performance. To ensure reliabledetection, the OSNR should be high enough to guarantee a threshold biterror rate (BER), which is typically 10⁻¹² for optical networks. Ingeneral, larger OSNRs are better; OSNRs less than or equal to one (0 dB)indicate that the noise power is greater than or equal to the signalpower. In embodiments of the disclosed planning tool, low OSNRs mayindicate the need for optical regeneration to maintain sufficiently lowBERs.

BER depends, in part, on the type of modulation used to encode thesignals. At present, optical networks support communications usingoptical signals encoded with a variety different modulation schemes, thechoice of which depends on the data rate, among other things. Networksthat support communications at data rates of 10 Gb/s or less typicallyuse on/off keying (OOK) schemes in which a bit is indicated by thepresence or absence of light. In other words, in OOK, turning the signalon indicates a “1,” and turning the signal off indicates a “0.”Unfortunately, OOK is not suitable for communications at higher datarates because optical impairments make it difficult to reliably detecthigh-speed signals modulated using OOK techniques.

Fortunately, phase-shift keying (PSK) techniques developed for wirelesscommunications can be used to increase the maximum data rate that can bereliably detected. With PSK modulation, the phase (e.g., 0°, 90°) of thereceived signal is compared to the phase of a reference signal to yielda bit value. Some PSK techniques, such as Quadrature PSK (QPSK), encodemultiple bits per symbol to achieve higher data rates for a givenbandwidth while maintaining a sufficiently low BER. (Even with phaseencoding, however, encoding more bits per symbol requires transmittingmore energy per signal to maintain a given OSNR.) PSK modulation is notsuitable for most optical communications because it is difficult todistribute stable reference signals throughout optical networks.

Differential PSK (DPSK) techniques, however, do not need referencesignals for accurate detection. In DPSK, the phase of the receivedsymbol is compared to the phase of the previously received symbol toproduce a difference in phase that corresponds to a bit value. Forexample, in differential QPSK (DQPSK), a 90° phase change between onesymbol and the next may correspond to a binary value of “01,” whereas a270° phase change may correspond to a binary value of “10.” Even thoughDQPSK is relatively well-known for wireless communications, it has notyet been adopted for widespread use in optical communications. Thus, fewpeople have contemplated problems associated with using DQPSK foroptical communications, including impairments due to transmitting DQPSKsignals over optical networks.

Because DQPSK uses phase, not intensity, to encode multiple bits persymbol, DQPSK modulation may lead to more severe optical impairments dueto four-wave mixing, self-phase modulation, and cross-phase modulation.(This is because DQPSK signals are at relatively constant power.) On theother hand, optical impairments due to PMD tend to be lower for DQPSKmodulation than for OOK modulation. As PMD usually impairs transmissionmore than four-wave mixing, self-phase modulation, or cross-phasemodulation, switching from OOK modulation to DQPSK modulation may reducethe aggregate impairment level, decreasing the BER for a givenmodulation rate at the expense of more complicated modulation anddetection.

As lower aggregate impairments lead to better signal fidelity,next-generation optical networks, such as networks that supportcommunications at 40 or 100 GB/s, may use DQPSK or similar modulationinstead of 00K. Because DQPSK modulation incurs different opticalimpairments than OOK modulation, however, planning networks that useDQPSK modulation requires considering factors not considered whenplanning networks that use OOK modulation. Planning becomes even morecomplicated for networks that support communication using both DQPSK andOOK modulation, as differently modulated signals interact with eachother to produce crosstalk and other optical impairments.

FIG. 1 is a schematic diagram that illustrates an example planning tool100 according to embodiments of the present invention. The planning tool100 may be used to plan the deployment of an optical networkcharacterized by a network topology and ONEs distributed throughout thetopology. The optical network may be organized in various topologies,including, but not limited to, ring, linear, and mesh topologies. ONEsmay include, but are not limited to, optical amplifiers, opticalregenerators, reconfigurable optical add/drop multiplexers (ROADMs), andwavelength-selective switches. ONEs may be located at or between nodesin the network depending on the network topology and the type of ONE.For example, ROADMs are typically located at network nodes to add and/ordrop signals, whereas amplifiers may be situated at or between networknodes to boost signal strength.

The planning tool 100 accounts for optical modulation type bycalculating optical impairments based on the selected type ofmodulation. For OOK, optical impairments due to PMD may dominate; forDQPSK, on the other hand, optical impairments due to self-phasemodulation may be larger than impairments due to PMD. The planning tool100 converts the calculated optical impairments to OSNR penaltiesassociated with a particular link or device. For example, PMD in a 100km long optical fiber may yield an OSNR penalty of 3 dB for OOKmodulation but only a 0.5 dB penalty for DQPSK modulation. Similarly,the frequency-dependent phase response of a filter in a ROADM may incura 1 dB penalty for DQPSK signals, but no penalty at all for OOK signals.

To use the planning tool 100, a user begins by initializing a model ofan optical network 120 using a user interface 105, such as a keyboard ora mouse, and a display 110. The model of the optical network 120includes information about network topology, such as the number andlocations of nodes, fiber types and lengths; models of ONEs, such asoptical amplifiers, optical regenerators, transponders, ROADMs; andmodels of optical signals 124, including models of the opticalmodulation. The various models 120, 122, and 124 may include noisefigures, losses, and other parameters relevant to calculating OSNR.

After the planning tool 100 initializes the model of the optical network120, it computes OSNR penalties throughout the model of the opticalnetwork 120 by calculating the effect of each optical component(including the optical fiber) on transmitted optical signals. Effectsmay be divided into: loss, which is the reduction of optical intensity;other OSNR effects, such as noise figure reduction due to amplification;and effects that do not cause OSNR penalties but should remain within agiven range to ensure proper network operation. These latter effects canbe tracked within any given network design and limited to the specifiedranges.

Amplifiers often have the most dramatic effects on OSNR because theyreduce the OSNR despite increasing signal strength. Other effects onOSNR include optical impairments due to chromatic dispersion and PMD,which are modeled as “optical penalties” or degradations to OSNR.Penalties include, but are not limited to: first-order PMD (also calleddifferential group delay); second-order PMD; self-phase modulation;cross-phase modulation; and crosstalk between channels, or four-wavemixing. These penalties depend on the bit rate of the signal (e.g., 10Gb/s, 40 Gb/s) and the optical modulation.

If the OSNR penalties are such that the OSNR for an optical signal fallsbelow a threshold for detectability, the planning tool adjusts the model120, then recomputes the OSNR penalties. For instance, the planning tool100 may replace a model of a noisy optical amplifier with a model of alow-noise amplifier to boost the OSNR; alternatively, it may add modelsof optical regenerators at models of nodes where the model signal 124 isbelow the detectability threshold. The planning tool 100 may also removemodels of amplifiers and models of regenerators where the OSNR is abovethe OSNR threshold to reduce network cost. In addition, the planningtool 100 may add models of regenerating lasers and receivers(transponders) between models of transmission end points (nodes) in themodel 120.

The planning tool 100 continues to adjust the model of the opticalnetwork 120 in an attempt to ensure that all the signals can be detectedwhile minimizing the cost of deploying the network. The planning tool100 may consider factors including loss in each span, expected trafficpatterns, and proposed regeneration locations along paths of expectedtraffic to determine locations for placing models of ONES 122 in themodel of the optical network 120.

The cost of a network may be determined as a function of the cost ofnetwork elements (e.g., regenerators and amplifiers) for maintainingsignal strength in the network. Cost can include installation time orexpense, maintenance time or expense, signal fidelity, networkredundancy, link availability, etc. In general, cost varies with thenumber and type of ONEs; usually, there is a trade-off between a largenumber of less expensive ONEs and a small number of more expensive ONEs.Further details on OSNR penalties and cost calculations for opticalnetworks can be found in U.S. application Ser. No. 12/228,763, U.S.application Ser. No. 12/387,023, and U.S. application Ser. No.12/436,397, all of which are incorporated herein by reference in theirentireties.

FIG. 2 is a flow diagram that illustrates optical network planning 200according to embodiments of the present invention. Planning begins withthe initialization (210) of a model of an optical network, includingmodels of ONEs, a model of the network topology, and models of opticalsignals with optical modulation. Planning continues with the computation(220) of OSNR penalties, which are based on both the type of opticalmodulation and optical impairments associated with the models of theONEs and the model of the network topology. The optical impairments mayalso depend on the optical modulation as described above. Next, planningproceeds with iterative adjustments (230) to the model of the opticalnetwork in an attempt to change (i.e., reduce) the computed OSNRpenalties (220) to enable detection of optical signals using knowndetection techniques. This may mean adjusting the model of the opticalnetwork so that the models of the optical signals are above a certainOSNR threshold at the nodes in the model of the optical network. Afterthe last iteration (240), indications of the model of the network arereported (250) to the user.

FIG. 3 is a flow diagram that shows optical network planning 300according to an alternative example embodiment of the present invention.Planning starts with initialization (310) of a model of optical network.In this alternative example embodiment, initialization of the model ofthe optical network (310) includes initialization of models ofamplifiers, optical regenerators, transponders, and ROADMs (312). Thesesub-models may incorporate noise figure, loss, phase response, and otherinformation relevant to OSNR penalty computation. Initialization (310)also includes selection of one or more types of optical modulation(314), such as OOK and/or DQPSK. Different types of optical modulationmay be selected for modeling in different parts of the model of theoptical network.

Planning progresses with computation of OSNR penalties (320) atlocations in the model of the optical network. The OSNR penalties arefunctions of optical impairments associated with various sub-modelswithin the model of the optical network. The optical impairments depend,in turn, on the particular optical modulation selected duringinitialization (314). For example, impairments due to PMD are lower forDQPSK than for OOK (322); in contrast, impairments due to four-wavemixing (4WM), self-phase modulation, and cross-phase modulation may belower for OOK than for DQPSK (324).

Once the OSNR penalties are computed (320), the model of the opticalnetwork can be adjusted to change the OSNR penalties. Adjustmentsinclude adding, removing, and repositioning nodes (332) to alter thetopology of the model of the optical network. In addition, models ofONEs may be added, removed, or replaced (334) with other ONEs. Forexample, a model of an optical amplifier may be replaced with a model ofan optical regenerator to reduce excessive OSNR penalties. That is,models of regenerating lasers and receivers (transponders) may be addedbetween models of transmission end points (nodes) in the model of theoptical network. If the OSNR penalties are small enough that the OSNR iswell above the threshold, then models of amplifiers may be removed orreplaced with models of noisier amplifiers; similarly, models of opticalregenerators may be removed or replaced with models of amplifiers.

Planning (300) may determine an optimum deployment pattern using anysuitable method. For example, planning (300) may rely on brute-forcecalculations, i.e., calculation of all possible arrangements of themodel of the optical network followed by selection of the optimumarrangement. The brute-force approach works with small numbers ofarrangements, but may require too many iterations and/or too much memoryfor large numbers of arrangements. Alternatively, the planning tool mayoptimize by minimizing (or maximizing) a figure of merit with aleast-squares estimation technique, such as the singular valuedecomposition method or the Levenberg-Marquardt method. The planningtool may also perform a search in a multidimensional space using thedownhill simplex (Nelder-Mead) method, Powell's method, simulatedannealing, genetic methods, or any other suitable method(s).

Planning (300) continues with iterations through the loop (320, 330,340) until the number of iterations equals a given number. At most, thenumber of iterations should be no more than the total number of possiblearrangements of the model of the optical network. Alternativeembodiments may iterate through the loop until the OSNR penaltiesconverge or until a given iteration time elapses. Still otherembodiments may stop iterating in response to user interruption,possibly in the form of a user-determined number of iterations. Theplanning tool may also iterate conditionally: for example, the planningtool may iterate through either a fixed number of iterations orconvergence of parameters associated with the model of the opticalnetwork, whichever occurs first. The foregoing examples are generallycovered by the term “number of iterations” herein.

After the last iteration, planning (300) terminates with reportingindications (350) of the model of the optical network to the user.Reporting indications (350) may include reporting the final models ofONEs and network topology (342), changes in the model of the opticalnetwork (344), or other parameters associated with the model of theoptical network. Indications may include visual displays or renderings,such as screen displays and printouts, auditory displays, orelectronically encoded representations, such as those stored oncomputer-readable media.

FIG. 4 is a block diagram that illustrates an example embodiment of anONE deployment planning apparatus 400 according to the disclosedplanning tool. The apparatus 400 can be used to plan the deployment ofan optical network by modeling the optical network and its performancefor different arrangements. The apparatus 400 includes an initializationunit 410, computing unit 420, adjustment unit 430, and reporting unit440. The initialization unit 410 initializes a model of the opticalnetwork being planned, including models that represent ONEs, ONEtopology, and optical modulation/signals. The computing unit 420calculates OSNR penalties for the initialized model as a function ofoptical impairments (e.g., insertion loss, propagation loss, amplifiernoise figure) and the optical modulation (e.g., OOK and DQPSK).

The adjustment unit 430 iteratively adjusts the model of the opticalnetwork to reduce the calculated OSNR penalties such that they fallbelow the threshold for signal detection. The adjustment unit 430 mayreplace models of ONEs with other models of ONEs or change the modeltopology. Once the adjustment unit 430 finishes with one iteration, itforwards the resulting network model to the computing unit 420 forrecalculation of the OSNR penalties. If the adjustment unit 430determines that no further iterations are desired, it forwards the finalmodel to the reporting unit 440, which outputs an correspondingindication of the final model to a user.

FIG. 5 is a block diagram that illustrates a second example embodimentof an optical network deployment planning apparatus 500 according to thedisclosed planning tool. Like the example shown in FIG. 4, the apparatus500 includes an initialization unit 510, a computing unit 520, anadjustment unit 530, and a reporting unit 540. These units 510, 520,530, 540 may be managed by a central processing unit (CPU) 570 thatoperates in conjunction with a random access memory (RAM) 572.

In the apparatus 500, the initialization unit 510 initializes a model ofan optical network using a group of ONE models 512 that includes modelsof generic ONEs 513, such as optical splitter/combiners, WDMs, opticalnetwork terminals, and optical line terminals. The group of ONE models912 also includes models of transponders 515, optical regenerators 516,ROADMs 517, and optical amplifiers 514, where the models of amplifiers514 include information about the amplifier operating settings, such asgain and noise figure. The model of the optical network includes a modelof ONE topology, which may be ring, mesh, linear, hybrid, or any othersuitable topology.

The model of the optical network also includes a model of the opticalmodulation(s) used to encode signals transported by the network. Theinitialization unit 510 selects one or more models of optical modulation523 from among a group of models 522. Example models include models ofOOK 524 and DQPSK 525. Different models of optical modulation may beused for different parts of the optical network. For example, DQPSK 525may be used to encode long-haul traffic, and OOK 524 may be used toencode metro traffic, or vice versa.

Once the initialization unit 510 completes initializing the model of theoptical network, the computing unit 520 calculates OSNR penaltiesthroughout the network. The computing unit 520 may refer to theoperating parameters of the ONEs, including gain settings of the modelsof optical amplifiers 516, during calculation of the OSNR penalties. Thecomputing unit 520 also uses information about the selected opticalmodulation to determine compute the OSNR penalties as described above.

After the computing unit 520 completes an initial OSNR penaltycalculation, the adjustment unit 530 determines whether or not to adjustthe model of the optical network. If the OSNR penalties fall withinaccepted limits, the adjustment unit 530 may determine that noadjustments are necessary. Otherwise, the adjustment unit 530 may adjustthe selection and/or distribution of ONE models 512, optical modulationmodels 522, and network topology. After finishing its adjustments, theadjustment unit 530 forwards the model of the optical network to thecomputing unit 520 for recalculation of the OSNR margin table.

The adjustment unit 530 may repeat this cycle for a fixed number ofiterations, where a counter 532 tracks the total number of iterationsand the current iteration number. The number of iterations may bepreset, fixed by the user, or equal to the total number of possiblearrangements of the model of the optical network. Alternatively, theadjustment unit 530 may halt the adjustment/recomputation loop inresponse to a user input 534 transmitted through an interface 574. Theadjustment unit 530 may also halt the adjustment/recomputation loop whenthe OSNR penalties converge. Conditional halts may also be used toterminate the adjustment/recomputation loop.

Once the adjustment/recomputation loop halts, the adjustment unit 530forwards indications of the final model to the reporting unit 540, whichreports the indication(s) via at least one of a variety of differentdisplay types 542 to a user. For example, the reporting unit 540 mayreport indications of the final selection of the ONE models, includingthe ONE location, type, and differences between the models asinitialized and the models as reported. In some embodiments, thereporting unit 540 may also report an ONE change display 548 indicatingany or all of the number, location, and changes in ONEs within themodeled optical network. The reporting unit 540 may also report the OSNRpenalties in tabular form 544, graphical form 546, or both.

It should be understood that the example flow diagrams of FIGS. 2 and 3can be readily converted to modules, subsystems, or systems that operatein a similar manner as set forth above. For example, the exampleembodiments may include an initialization module, computing module, andreporting module.

It should be further understood that the examples presented herein caninclude more or fewer components, be partitioned into subunits, or beimplemented in different combinations. Moreover, the flow diagramsherein may be implemented in hardware, firmware, or software. Ifimplemented in software, the software may be written in any suitablesoftware language. The software may be embodied on any form of computerreadable medium, such Random Access Memory (RAM), Read-Only Memory(ROM), or magnetic or optical disk, and loaded and executed by genericor custom processor(s).

While this invention has been particularly shown and described withreferences to example embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A method of planning deployment of an opticalnetwork, the method comprising: enabling modeling of an optical network,including a model representing a topology of optical network elements(ONEs), models of ONEs, and models of optical signals with opticalmodulation, and enabling selection of an optical modulation to be usedto support communications between respective ONEs on the opticalnetwork; computing optical signal-to-noise ratio (OSNR) penalties as afunction of the optical modulation selected; iteratively adjusting themodel of the optical network to change the OSNR penalties in a mannerknown to enable detection of the optical signals to support thecommunications between the ONEs; and reporting indications of the modelof the optical network.
 2. The method of claim 1 wherein enablingmodeling of the optical network includes enabling selection of an on/offkeying optical modulation or a differential quadrature phase-shiftkeying optical modulation to be used to support communications betweenrespective ONEs on the optical network.
 3. The method of claim 1 whereinenabling modeling of the optical network includes models of any ofoptical amplifiers, transponders, optical regenerators, andreconfigurable optical add/drop multiplexers.
 4. The method of claim 1wherein computing the OSNR penalties includes computing the OSNR as afunction of the optical modulation selected and as a function of opticalimpairments associated with the model of the optical network.
 5. Themethod of claim 1 wherein computing OSNR penalties includes calculatingoptical impairments due to any of chromatic dispersion, polarizationmode dispersion, four-wave mixing, self-phase modulation, andcross-phase modulation.
 6. The method of claim 5 wherein computing OSNRpenalties includes calculating lower optical impairments due topolarization mode dispersion for differential quadrature phase-shiftkeying than for on/off keying and calculating higher optical impairmentsdue to four-wave mixing, self-phase modulation, and cross-phasemodulation for differential quadrature phase-shift keying than foron/off keying.
 7. The method of claim 1 wherein iteratively adjustingthe model of the optical network includes: replacing models of a firsttype of ONE with models of a second type of ONE; adding or removingmodels of nodes from the topology of ONEs, including adding models ofregenerating lasers and receivers between transmission end points;bounding the OSNR penalties to remaing below a threshold throughout themodel of the optical network; or enabling any combination of thereof. 8.The method of claim 1 wherein reporting indications of the model of theoptical network includes reporting indications of the topology of ONEsindications of the ONEs.
 9. The method of claim 1 wherein reportingindications of the model of the optical network includes reporting ofchanges in the topology of ONEs and indications of changes in the ONEs.10. The method of claim 1 wherein reporting indications of the model ofthe optical network includes reporting after a given number ofiterations, the number of iterations being selected from: a fixednumber, a convergence of at least a subset of the OSNR penalties, anumber performed in a period of time, and a user-determined number. 11.An apparatus to plan deployment of an optical network, the apparatuscomprising: a modeling unit configured to enable a model of an opticalnetwork including a model representing a topology of optical networkelements (ONEs), models of ONEs, and models of optical signals withoptical modulation, and to enable selection of an optical modulation tobe used to support communications between respective ONEs on the opticalnetwork; a computing unit configured to compute optical signal-to-noiseratio (OSNR) penalties as a function of the optical modulation selected;an adjustment unit configured to iteratively adjust the model of theoptical network to change the OSNR penalties in a manner known to enabledetection of the optical signals to support the communications betweenthe ONEs; and a reporting unit configured to report indications of themodel of the optical network.
 12. The apparatus of claim 11 wherein themodeling unit is further configured to enable selection of an on/offkeying optical modulation or a differential quadrature phase-shiftkeying optical modulation to be used to support communications betweenrespective ONEs on the optical network.
 13. The apparatus of claim 11,wherein the modeling unit is further configured to enable models of anyof optical amplifiers, transponders, optical regenerators, andreconfigurable optical add/drop multiplexers.
 14. The apparatus of claim11 wherein the computing unit is further configured to compute the OSNRpenalties as the function of the optical modulation selected and as afunction of optical impairments associated with the model of the opticalnetwork.
 15. The apparatus of claim 11 wherein the computing unit isfurther configured to calculate optical impairments due to any ofchromatic dispersion, polarization mode dispersion, four-wave mixing,self-phase modulation, and cross-phase modulation.
 16. The apparatus ofclaim 15 wherein the computing unit is further configured to calculatelower optical impairments due to polarization mode dispersion fordifferential quadrature phase-shift keying than for on/off keying, andto calculate higher optical impairments due to four-wave mixing,self-phase modulation, and cross-phase modulation for differentialquadrature phase-shift keying than for on/off keying.
 17. The apparatusof claim 11 wherein the adjustment unit is further configured to:replace models of a first type of ONE with models of a second type ofONE; add or remove nodes from the topology of ONEs, including addingregenerating lasers and receivers between transmission end points of theoptical network; bound the OSNR penalties to remain below a thresholdthroughout the model of the optical network; or any combination thereof18. The apparatus of claim 11 wherein the reporting unit is furtherconfigured to report indications of the topology of ONEs and indicationsof the ONEs.
 19. The apparatus of claim 11 wherein the reporting unit isfurther configured to report indications of changes in the topology ofONEs and indications of changes in the ONEs.
 20. The apparatus of claim11 wherein the reporting unit is further configured to report after agiven number of iterations, the number of iterations being selectedfrom: a fixed number, a convergence of at least a subset of the OSNRpenalties, a number performed in a period of time, and a user-determinednumber.
 21. A computer program product including a computer-readablemedium having a computer-readable program, the computer-readableprogram, when executed by a computer, causes the computer to: model anoptical network including enabling a model representing a topology ofoptical network elements (ONEs), models of ONEs, and models of opticalsignals with optical modulation, and selection of an optical modulationto be used to support communications between respective ONEs on theoptical network; compute optical signal-to-noise ratio (OSNR) penaltiesas a function of the optical modulation selected; iteratively adjust themodel of the optical network in an attempt to change the OSNR penaltiesin a manner known to enable detection of the optical signals to supportcommunications between ONEs; and report indications of the model of theoptical network.